1. Field of the Invention
The present invention relates to a recording/reproducing apparatus and method of an optical recording medium, and in particular to an apparatus and a method for detecting a tilt of an optical recording medium and compensating a quantity and a direction thereof.
2. Description of the Background Art
Generally, optical recording mediums are classified into a ROM-type (Read Only Memory) medium from which data can be only read, a WORM-type (Write Once, Read Many) medium into which data can be only recorded a single time and from which recorded data can ben read many times, and a rewritable type medium, for example a CD-RW, and a rewritable digital versatile disk (DVD-RW or DVD-RAM), into/from which data/recorded data can be repeatedly recorded/reproduced.
The rewritable optical disk has a land in which information signals can be recorded and a groove in which any information signals can not be recorded, but recently the rewritable optical disk, for example DVD-RAM, can record information signals on the tracks of both the land and groove in order to record more information signals to the disk. Therefore, the wavelength of a laser beam from an optical pickup unit for recording and reproducing the signals to/from the disk must be shorter, and the size of the laser beam must be reduced as numerical aperture of an object lens for collecting light is expanded.
As described above, in the rewritable optical disk, in order to increase the recording density, the distance between the signal tracks must be narrowed. In other words, a signal track pitch is narrowed.
The rewritable high-density optical disk may have a distortion caused during the process of injection and hardening resin. Namely, although a center hole of the disk is pierced into the disk, an eccentricity of the disk may occur. When a disk track is recorded accurately according to a predetermined standard pitch as a spiral value, an eccentricity occurs due to declination of the center hole. Accordingly, the center shaft of a spindle motor does not coincide with the center of the track, although the disk rotates with eccentricity.
Due to the problems above, it is difficult to read out a signal from a target track accurately. CD (Compact Disk) and DVD (Digital Versatile Disk) types have a declination standard and are recorded/reproduced signals onto/from themselves by a tracking servo of the optical recording medium recording/reproducing apparatus against eccentricity to make an optical beam trace a target track any time.
The tracking servo generates al electric signal corresponding to a beam trace state, corrects the position of the beam outputted from the object lens of the optical pickup unit in a radial direction based on the electric signal, and thereby controls the optical pickup unit to trace the track accurately.
Meanwhile, de-track of the beam occurs due to not only the eccentricity of a disk, but also a slant in the disk. The slant may occur due to declination problem of machinery caused in the process of installing the disk at a spindle motor and distortion of the disk. In this case, a focusing signal and a specula surface of the disk do not cross accurately at right angles, but swerves. Likewise, a type of disk slant is tilt.
The tilt is no real problem with CDs having a big tilt margin due to broad track pitch. Herein, the tilt margin means correctable tilt quantity when the disk is declined at a certain degree. But, an application apparatus like an optical disk has a high density, in a DVD having a narrow track pitch, when a slight tilt occurs, in other words, when a disk declines slightly, a beam escapes on an other track, namely, de-track can occur due to a small radial tilt margin for jitters, the tracking servo is not enough to compensate in this case. A beam escapes to a next track by tilt, and does not exist at the center of a track, but the tracking servo may misjudge that the beam had traced a track accurately. At this time, data can not be recorded at a target track and can not be reproduced accurately, thus, double perversion occurs when the recorded data is reproduced.
To solve the above-mentioned tilt problem, there is an exclusive tilt sensor having a light receiving element separately for detecting tilt of a disk. But, its efficiency is low and the size of the set is a problem.